Interface circuits are used to transmit and receive electrical signals between devices in electronic systems. These systems include digital systems where the signals communicated between the devices transition between high and low voltage levels. In some of these systems electrical signals need to be transmitted between integrated circuits (ICs). Interface circuits are used to minimize the effects of electrical interference, such as noise, that interferes with signal transitions between logic levels. They are also helpful to compensate for weak signal line drivers.
Interface circuits designed to sense the signal transitions on input signal lines provide improved signal communication. As an input signal approaches a midpoint between the high and low voltage levels, an interface circuit senses the transition and provides “clean” signal transitions to the circuits internal to the ICs by driving the inputs of the internal circuits to either the high or low level. Typically these interface circuits consist of amplifiers that sense the voltage transitions. A sense amplifier compares an input signal to a voltage reference. The comparison is done using a differential pair at the input of the amplifier. The output of the amplifier transitions when the input signal equals the voltage reference.
A general trend in IC design is to reduce system power consumption in part by reducing the power supply voltages. Variations in the difference between the high supply voltage and the transition voltage can occur among wafer runs due to variations in the fabrication process. If the variations result in an offset that causes the transition voltage to move towards the higher voltage supply rail and the cause the higher voltage supply rail to move towards the transition voltage, the system becomes more susceptible to spurious transitions from noise. This is especially important when supply voltages are below the two-volt level.
Some designers have approached the problem by tuning the amplifier stages to a specific transition voltage and supply voltage. However, if a different supply voltage is desired, the circuits need to be redesigned to accommodate the new voltage reference and transition voltage levels. What is needed is an interface circuit design with a transition voltage that accommodates changes in the supply voltage.